Heartnut (Juglans ailantifolia var. cordiformis) is a type of walnut with a heart-shaped nut. All heartnuts are of the Japanese walnut species, or hybrids there of. The heartnut is a genetic “sport” or mutation of the normal, wild type nut of Japanese walnut.
Normal Japanese walnuts are cylindrical, elongated, rough or spiny, very thick-shelled and difficult to crack. In contrast, the heartnut is (more or less) Valentine-heart shaped and somewhat flattened. The heartnut is the easiest of all walnuts to husk and to crack. When moderate pressure is applied across the “shoulders” of the heart, the nut “pops” in two along the suture line, yielding two valentine-heart shaped halves. In cultivars with only “fair” to “good” cracking quality, the kernel breaks in half and comes out in two pieces. In the best cultivars, the kernel falls free from the shell in a single, unbroken piece – the only walnut in the world with this ability. On top of that, the heartnut is also the sweetest and mildest tasting of all the walnuts, beating out the so-called “English” walnut in taste tests every time. As if that weren’t enough, the heartnut has an incredible ability to store at room temperature.
The flavor actually develops in storage and peaks between five and eight years. Beyond 10 years at room temperature, heartnuts may become slightly stale, but not enough to keep them from being enjoyable.
The heartnut tree itself is incredibly fast growing. With adequate moisture and protection from weed competition they can put on six to eight feet of height growth per year in Iowa. The twigs are often as big around as broom handles. Leaves are compound and can be two to three feet long, with a very “tropical” look. Though the trees can become quite large, they tend to be low and spreading, like a live-oak. An old tree with a three foot diameter trunk and a 100 foot spread may be only 20 feet to 30 feet tall.
Heartnuts prefer moist, fertile, well-drained soil, though they will tolerate somewhat poorly drained soil better than most fruit and nut trees. They are best adapted to zones 5 and 6, with some of the hybrids suited through zone 4.
Superior heartnut cultivars can be grafted onto black walnut rootstock. The resulting tree can be twice as productive as seedling heartnuts or heartnuts grafted onto other heartnut rootstocks. Heartnuts are among the most challenging trees to graft, requiring great skill for success. For this reason, grafted heartnuts are very difficult to find in the nursery trade. More common are heartnut seedlings. The problem with seedlings is that they may grow into trees that bear nuts with no resemblance to those of their parents. Many heartnut seedlings “revert back to wild-type” – that is, their nuts will resemble the thick-shelled hard –to-crack wild Japanese walnuts. Some heartnut parent trees produce as much as 70% of offspring that grow into wild-type trees. On the other hand, the best heartnut parents will produce up to 90% heartnut offspring,
especially if they are pollenized by other good heartnut parents. If you are planting heartnut seedlings, you should plant about twice as many as the number of trees you want to end up with – that way, you can grow them up to bearing size, select the ones with the best nuts, and cull the poor ones.
Heartnuts are highly susceptible to walnut bunch disease. The disease is caused by a phytoplasma organism, and it’s transferred from tree to tree by leaf-hoppers. The disease causes “witches broom” type growth on the tree, and is frequently fatal. The good news is that the disease is apparently strongly linked to zinc deficiency in the soil. A foliar feed of the naturally occurring mineral zinc sulfate, applied in the spring, will usually clear up the disease in a single growing season. Zinc sulfate is applied to the soil around the drip line of the tree to prevent the disease from coming back.
Heartnuts have some great potential as a commercial tree. The unique shape of the nut alone makes them very attractive to consumers who eagerly pay $6 to $7 per pound for them. Add to this their rapid growth, heavy bearing, ease of husking and cracking, and great taste, and you have a tree that is well worth growing as a backyard tree or for a serious commercial venture. – Tom Wahl